This invention relates to two-cycle internal combustion engines. Such engines are used in a very wide variety of applications, for example, lawn mowers, string trimmers, snow and leafblowers, personal watercraft such as "jet-skis", motorcycles, boats and chain saws. Such engines are typically lubricated by means of oil mixed with raw gasoline in some fixed ratio, for example 50 parts of gasoline to 1 part oil instead of an oil-filled crankcase, as in four-cycle engines. The term "two-cycle" is actually a shortened version of "two-stroke cycle", and refers to the fact that each cycle of the engine is completed in two strokes of the piston--one up and one down. The engine must be designed so that the combustion chamber can be supplied with a fresh charge of fuel when the piston is in the extreme outward position.
In most two-cycle engines, valves are replaced by ports in the cylinder, through which fuel is drawn into the cylinder and exhaust gases are expelled. These engines are called "piston-ported" engines. One cylinder port, the exhaust port, leads to the atmosphere and permits exhaust gases to be expelled from the combustion chamber by the pressure within the cylinder. The other cylinder port, the fuel intake port, opens from a bypass to the crankcase, and through it a slightly compressed charge of fuel is delivered to the cylinder when the piston uncovers the port. In some types of two-cycle engines, the gas is compressed in the crankcase by the piston, but in others an external compressor of a piston or rotary type is employed. The principles of this invention have application not only to piston-ported engines but also to reed-valve engines, and to both air-cooled and water-cooled two-cycle engines.
The fuel induction side of the piston is adequately lubricated by the intake of outside air and fuel vapor. On the other hand, the exhaust side of the piston is exposed to extremely high friction which can easily cause scoring or other damage to the piston and cylinder walls on the exhaust side. This is the limiting factor in lubricating two-cycle engines, and the oil in the oil/gasoline mixture is intended to provide lubrication primarily to the exhaust side of the piston and cylinder, as well as the piston pin and crankshaft.
Mixture of lubricating oil with gasoline in such engines creates a number of problems. The lubricating oil is only partially burned during the ignition and combustion. The remaining oil is exhausted as a blue/gray exhaust stream which has an foul odor, creates air pollution, and coats environmental surfaces with which it comes into contact. Some amount of the oil remains in the engine and eventually fouls the spark plug, cylinder walls and piston components. The oil component of the gasoline/oil mixture is also quite expensive in relation to the ratio used, and adds an inordinate amount to the overall fuel costs. These disadvantages have caused more expensive, heavier and lower efficiency four-cycle engines to be used for many applications where a two-cycle engine could otherwise be used.
Experimentation by applicant has shown that conventional two-cycle engines can be run for extended periods of time with a gasoline-only fuel without damage to the fuel induction side of the piston or cylinder. Raw gasoline has a high degree of lubricity in its liquid and gaseous state, and thus by itself can provide adequate lubrication to those areas with which it comes in contact. Damage in such two-cycle engines is principally to the exhaust side of the engine, and is caused by lack of lubrication. Thus, if additional gasoline-only lubrication could be supplied to the exhaust side of the engine and to the top of the inside of the piston without introduction of lubricating oil into the gasoline, two-cycle engines would run cleaner and more efficiently.